We have previously shown that co-culture of T cell enriched spleen lymphocytes can reduce collagen synthesis and stimulate proliferative activity by liver fibroblasts from Schistosoma mansoni infected mice. The present study examines which subset of T cells is responsible for this modulation. Co-culture of fibroblasts with the total T cell population lead to significant stimulation in fibroblast proliferative activity and a significant decrease in the incorporation of 14C-proline into collagenase-sensitive protein. There was virtually no effect on total incorporation of 14C-proline in non-collagen proteins. An additional significant increase in fibroblast proliferative activity and another significant decrease in collagen synthesis accompanied by a 2-fold increase in non-collagen protein production was noted when fibroblasts were co-cultured with Lyt-1+ cells. Co-culture of fibroblasts with Lyt-2+ cells did not differ significantly from co-culture with total T cells. Mitomycin treatment of the lyphocytes blunted their specific effects, suggesting that proliferation of T cells is required for maximal exertion of their regulatory activity. These results suggest that the T cells which mediate these effects belong to the Lyt-1+ helper class and are distinct from the Lyt-2+ suppressor cells.